This invention relates to an antenna feed system for communication techniques utilizing dual orthogonal polarization, for example, a horizontally and a vertically polarized wave for linear polarization or a righthand and a lefthand, circularly polarized wave for circular polarization, and more particularly to an antenna feed system for compensating for a cross polarization due to the depolarization effect of the propagation passageway therefor.
There are well known frequency reuse systems employing two orthogonally polarized waves of the same frequency spectrum carrying different information thereby. Such frequency reutilizing systems are introduced into satellite communication systems and others. In those systems it is essential to sufficiently compensate for the cross polarization due to the depolarization effect of propagation passageways. To this end, there have been previously proposed various approaches, one of which is disclosed in Japanese laid-open patent application No. 115,717/1976 entitled "Cross Polarization Compensating System". The disclosed system comprises a simultaneously operated transmitting and receiving antenna and a transmitter unit and a receiver unit connected to the antenna through a duplexer. In the receiver unit, an ortho-mode transducer resolves a pair of received elliptically polarized oppositely rotated waves into a pair of orthogonal components after the elliptically polarized waves have be transformed so as to have respective major axes orthogonal to each other. Those orthogonal components are applied via a righthand and a lefthand circular polarization path to respective branching circuits to be halved. One halved portion of each orthogonal component is supplied to a transfer circuit and the other halved portion thereof is supplied to a coupler.
Each transfer circuit is connected to that coupler supplied with the other halved portion of the other orthogonal component and adjusts the phase angle and amplitude of the halved portion of the associated orthogonal component so that both couplers produce the desired signals free from the influence of a cross polarization.
In the transmitter unit, a pair of linearly polarized orthogonal waves are supplied to respective branching circuits to be halved. One halved portion of each of the linearly polarized orthogonal waves is supplied to a transfer circuit equivalent to that included in the receiver unit and the other halved portion thereof is supplied to a coupler. The transfer circuits impart preliminarily to halved portions of the linearly polarized orthogonal waves a cross polarization sufficient to cancel the depolarization of propagation passageways observed at an associated receiving point when the antenna transmits an electromagnetic wave in the form of a pair of elliptically polarized waves oppositely rotated through components corresponding to those disposed in the receiver unit.
The cross polarization compensating system as described above has been disadvantageous in that a pair of transfer circuits are required to be disposed in each of the transmitter and receiver units, resulting in a complicated, large-sized circuit configuration. Therefore, the cited system has been expensive to be manufactured and decreased in reliability. Also, an improvement in cross polarization has been affected by the unbalanced characteristics of the amplitude and phase angle developed between the righthand and lefthand circular polarization paths in each of the transmitter and receiver units. Furthermore, the pair of transfer circuits have required control mechanisms for adjusting the cross polarization components necessary for the cancellation, resulting in the disadvantage that failures occur with a high probability.
Accordingly, it is an object of the present invention to provide a new and improved antenna feeding system requiring only a single transfer circuit in one of either a transmitter and a receiver unit.
It is another object of the present invention to provide a new and improved antenna feed system free from the influence of the unbalanced characteristics of the amplitude and phase angle upon a righthand and a lefthand circular polarization path. It is still another object of the present invention to decrease the number of control mechanisms forming the transfer circuits to reduce the probability of the occurrence of failures.